The present invention relates to a magneto-optic recording medium and more specifically to a magneto-optic recording medium which can perform Magnetically Induced Super Resolution (MSR) reproduction.
Magneto-optic disks are known as a high-density recording medium and are expected to have a higher-density recording capacity in response to an increase in the amount of data. Higher density can be realized by shortening the intervals of the recording signals (marks), but the recording and the reproduction are restricted by the size (spot diameter) of the light beam on the medium. In order to reproduce a small mark whose period is equal to or smaller than the spot diameter, the spot diameter can be reduced. However, this reduction is restricted by the wavelength .lambda. of the light source and the numerical aperture NA of the object lens. Consequently, it has been difficult to reproduce a recording mark which is more minute than the resolution of the optical system.
Recently, magnetically induced super resolution (MSR) media which can bring about the same effects as reducing the spot diameter by making a multi-layered recording medium and utilizing the temperature distribution of a medium formed in a beam spot have been suggested (Japanese Patent Application Laid-Open Nos. 1-143041 (1989), 3-93058 (1991), 4-271039 (1992), and others).
The MSR medium suggested in Japanese Patent Application Laid-Open No. 1-143041 (1989) can reproduce a mark smaller than the spot diameter without providing an initializing magnet, by applying a magnetic field of about several hundred Oe on reproducing. However, the detection area is too large to reduce the truck pitch, so that it is disadvantage to a densification in the direction of a disk radius. The MSR medium suggested in Japanese Patent Application Laid-Open No. 3-93058 (1991) requires an additional initializing magnet of about 3.4 to 4 kOe, and has a problem that the detection area is enlarged in accordance with an increase in the power of the reproduction beam, although it has a smaller detection area than Japanese Patent Application Laid-Open No. 1-143041 (1989). The provision of a magnet of several kOe makes it difficult to reduce the size of the recording/reproducing device. The MSR medium suggested in Japanese Patent Application Laid-Open No. 4-271039 (1992) has a small detection area regardless of the power of the reproduction beam, and can reproduce a mark recorded with a high resolution in the direction of the disk radius. However, there is a problem that an initializing magnet of several kOe is needed in addition to a reproduction magnetic field of several hundred Oe.
In order to solve these problems, the applicant of the present application has suggested a magnet-optic medium in Japanese Patent Application Laid-Open No. 7-244877 (1995) which can realize MSR reproduction by a RAD double mask method, by applying a reproduction magnetic field as low as several hundred Oe, without using an initializing magnet. FIG. 1 shows the magnetized condition of the reproduction of the MSR medium which has been disclosed by the applicant of the present application, and a film structure. As shown in FIG. 1, a magneto-optic disk 3 comprises a reproduction layer 33, a control layer 34, and a recording layer 35 accumulated in this order on a substrate (not shown). The reproduction layer 33 is a transition metal magnetization dominant film and has an easy axis of magnetization in the perpendicular direction, that is, the direction in which the layers are deposited. The control layer 34 is a rare-earth magnetization dominant film and has an easy axis of magnetization in the in-plane direction at room temperature (10.degree. C. to 35.degree. C.). The easy axis of magnetization changes from the in-plane direction to the perpendicular direction when the temperature reaches a predetermined temperature higher than the room temperature. The recording layer 35 is a transition metal magnetization dominant film and has an easy axis of magnetization in the perpendicular direction.
The magneto-optic disk 3 having such a structure is provided with a mark which is recorded in the downward direction, and when this mark is reproduced, a reproduction laser beam is irradiated while a reproduction magnetic field is being applied. The magnet-optic disk 3 generates a temperature distribution in the laser spot S, and the magnetization direction of the recording layer 35 is masked in a high temperature area (front mask) and a low temperature area (rear mask), and the mark is read from an intermediate temperature area (opening). FIG. 1 shows the case where the reproduction magnetic field is applied in the upward direction, that is, the direction for erasing data. Such a film structure and a reproducing principle of the MSR medium are detailed in Japanese Patent Application Laid-Open No. 7-244877 (1995) and Japanese Patent Application No. 8-276672 (1996), so that their description will be omitted.
According to the MSR medium which forms a mask in the beam spot S, the beam spot S has a slightly different mask formation area depending on the direction in which the magnetic field is applied in order to reproduce. Such a difference in the mask formation area makes the edges before and after the waveform of the reproduction signal (reproduction waveform) have different inclination from each other. FIGS. 2A and 2B show the waveforms of the reproduction signals which are obtained by applying magnetic fields in the erasing direction and in the recording direction on the magneto-optic disk 3 obtained by the applicant of the present application.
As shown in FIG. 2A, when reproduction is performed by applying the magnetic field on the MSR medium in the erasing direction, the inclination of the front edge of the reproduction waveform is less steep than that of the rear edge, so that the front edge has a larger jitter than that of the rear edge against the same strength of noise power. Consequently, the quality of the reproduction signal is affected by the jitter of the front edge. On the other hand, as shown in FIG. 2B, when reproduction is performed by applying the magnetic field on the MSR medium in the recording direction, the inclination of the rear edge of the reproduction waveform is less steep than that of the front edge, so that the quality of the reproduction signal is affected by the jitter of the rear edge. In either case, the reproduction waveform has nonlinearity, and the edge on the side where the magnetization direction of the reproduction layer accords with the erasing direction is more steep. Since the jitter increases on the side where the edge is less steep, it is difficult to perform correct detection of reproduction data.
In order to solve this problem, the applicant of the present invention has disclosed a method of detecting the timing of an edge of a reproduction waveform based on the obtained reproduction signal and then reversing (modulating) the reproduction magnetic field after the detection of the edge. According to this reproduction method, when the front edge and the rear edge of the recording mark are outputted, the magnetic field in the direction of the steeper edge can be applied, so that both edges of the reproduction waveform can be closer to be symmetrical. However, this method has a problem that the strength of the magnetic field is insufficient when the modulation frequency of the reproduction magnetic field is high. FIG. 3 is a graph showing the relationship between the frequency of the applied magnetic field and the intensity of the magnetic field according to the reproduction method disclosed by the applicant of the present invention. The ordinate axis indicates the intensity of the magnetic field and the abscissa axis indicates time. As shown in this graph, when the modulation frequency of the reproduction magnetic field is higher, the intensity of the magnetic field is low. There has been a problem that when the intensity of the magnetic field is low, it is hard to detect the reproduction signal correctly. Furthermore, it is necessary for a device for executing this reproduction method to provide an additional driving circuit for modulating the reproduction magnetic field.